The present invention relates to a fluid-pressure detector, and more particularly to a disposable fluid-pressure detector used in a fluid-circulating line.
As an example of this kind of fluid-circulating line, there is known a blood line provided in a so-called dialysis system or plasma-separating system. In such blood line, there has been generally used a pressure-detecting chamber 10 as a fluid-pressure detector for detecting the pressure of flowing blood FIG. 4 shows a schematic structure of the pressure-detecting chamber 10.
The pressure-detecting chamber 10 has a tubular closed container 11 made of synthetic resin such as soft vinyl chloride. Tubes 12 for flowing blood therein are connected to both upper end and lower end of the closed container 11. A monitoring tube 13 is also connected to the upper end of the closed container 11. A pressure gauge (not shown) is attached to an opening end of the monitoring tube 13. That is, in this pressure-detecting chamber 10, blood which flows in the tube 12 is contained in the lower portion of the closed container 11, and the change in pressure of air in a closed room 14 over the blood B contained in the closed container 11 is detected by means of the pressure gauge. Thus, the pressure of blood flowing in the tube 12 is detected through the pressure of air in the closed room 14.
In the above-mentioned pressure-detecting chamber 10, a pressure-detecting portion communicates with the inside of the chamber since the blood pressure is detected through the air in the closed container 11. Accordingly, it is necessary to provide means for preventing bacteria such as a filter for preventing bacteria between the pressure-detecting portion and the chamber 10. Further, it is necessary to provide means for detecting a blood level in the chamber 10 since there is a danger that air in the chamber 10 enters the blood line by the continuous negative pressure. Still further, the chamber 10 is a relatively large part having a volume of 10 to 30 ml, so that there is a disadvantage that the placement or positioning of the chamber requires a large space.
In order to solve the above-mentioned disadvantages, there is proposed a fluid-pressure detector of which side sectional view is shown in FIG. 5. The fluid-pressure detector has a pillow-like body 21 which is made expandable. Tubes 22 for flowing blood which constitutes a blood line are connected to both ends of the pillow-like body 21. A detecting means 23 such as a strain gauge capable of detecting the amount of expansion or shrinkage of the pillow-like body 21 is fixedly placed outside the pillow-like body 21. A detecting portion 23a of the detecting means 23 is so arranged as to directly contact the outer surface of the pillow-like body 21. Since the blood line is discarded after being used one time, the pillow-like body 21 connected to the blood line and the expensive detecting means 23 are not connected to each other, i.e. are separately placed. According to the fluid-pressure detector 20, there can be effectively solved disadvantages caused by detecting blood pressure through air as in the above-mentioned pressure-detecting chamber 10.
In the fluid-pressure detector 20, the pillow-like body 21 and the detecting means 23 are not connected to each other Accordingly, the pressure of blood can be accurately detected so long as the pressure is positive. However, the continuous and accurate detection of the pressure of blood cannot be carried out since the outer surface of the pillow-like body 21 comes off from the detecting portion 23a of the detecting means when the pressure of blood becomes negative, whereby the pillow-like body 21 shrinks beyond the projection limit of the detecting portion 23a of the detecting means 23. Negative pressure can of course be detected if the pillow-like body 21 and the detecting portion 23a of the detecting means 23 are connected to each other. In that case, however, the detecting means 23 is discarded when the blood line including the pillow-like body 21 is discarded. The discard of expensive detecting means 23 is very uneconomical.
The present invention was made to solve the above-mentioned drawback, and it is an object of the present invention to provide a fluid-pressure detector capable of improving the accuracy of detection and reducing the space required for the placement of the detector. The detector of the present invention further enables continuous and accurate detection of the fluid pressure even when the fluid pressure becomes negative, and enables the discard of the pillow-like body connected to the fluid-circulating line with leaving the detecting means as it is.